Assertion (A) : Dual slope (A/D) converter is the most preferred conversion technique employed in most of the digital multimeters. Reason (R) : Dual slope (A/D) converter provides high accuracy while at the same time suppresses the HUM effect on the input signal.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both are true, and the reason names precisely the two properties that won the dual-slope converter its place in multimeters — option 1.
How it works. The conversion has two phases. In the first, the unknown input is integrated for a fixed time \(T_{1}\). In the second, a reference of opposite polarity is integrated until the integrator returns to zero, taking a time \(T_{2}\) that is counted. Then
\(V_{in}=V_{ref}\dfrac{T_{2}}{T_{1}}\)
Why the accuracy is high — the components cancel. The same integrating capacitor, the same resistor and the same clock are used in both phases, so R, C and the clock frequency all cancel out of the final ratio. Their absolute values, their tolerance, their temperature drift and their ageing are therefore irrelevant. Almost no other architecture has this property: a successive-approximation converter is only as good as its internal DAC, and a flash converter only as good as its resistor ladder. Accuracy here depends on the reference alone.
Why hum is rejected — and this is the decisive point. Integrating a signal over a time \(T_{1}\) averages it, and the average of any sinusoid over a whole number of its own periods is exactly zero. Choosing
\(T_{1}=n\times\dfrac{1}{f_{mains}}\)
— typically 100 ms, which is five cycles of 50 Hz — makes the converter completely blind to mains interference picked up on the leads. The rejection is theoretically infinite at 50 Hz and its harmonics, and instruments sold in both markets use 100 ms so as to be a whole number of cycles at 50 and 60 Hz. This is called normal-mode rejection, and no sampling converter can achieve it, because a sample taken at an instant captures whatever the hum happens to be doing then.
| Dual slope | Successive approximation | |
|---|---|---|
| Speed | Slow — a few per second | Fast — microseconds |
| Accuracy limited by | The reference only | The internal DAC |
| Hum rejection | Excellent | None inherently |
The trade is entirely acceptable in a multimeter, where a reading two or three times a second is all a human can use, and where accuracy and quiet readings are everything.
Hence, both (A) and (R) are true and (R) is the correct explanation of (A).
Similar Questions
In successive approximation converter input to the comparator is through
Assertion (A) : Dual-slope A/D converter is most preferred A/D conversion approach in digital multimeters.
Reason (R) : Dual-slope A/D converter provides high accuracy in A/D conversion, while at the same time suppressing the hum effect on the input signal.
A dual slope A to D converter has the following :
(a) a differentiator
(b) a comparator
(c) an integrator
(d) a binary counter
Out of above, following is correct :
For A to D converters, following statements are given :
(a) It is the process of converting time varying input to discrete output levels.
(b) Flash type A/D conversion are slow in nature.
(c) The quantisation error depends upon the step size of quantisation.
(d) Flash type A/D conversion is a simultaneous conversion.
Which of the above are correct ?
Assertion (A) : The output of an 8 bit A to D convertor is 80H for an input of 2.5V.
Reason (R) : ADC has an output range of 00 to FFH for an input range of −5V to +5V.
Select your answer using the codes given below.
Which of the following A/D convertor is fastest ?
The conversion time for an 8-bit successive approximation ADC which is driven by 1-MHz clock is :
An analog voltage in the range 0 to V volts is to be converted into 3 bit digital output. It is divided into eight intervals. The top and bottom intervals are \(\dfrac{V}{14}\) and the middle six intervals are \(\dfrac{V}{7}\). The maximum quantization error will be :
List – I List – II a. Flash converter i. Integrating type b. Successive approximation ii. Fastest converter c. Counter Ramp iii. Maximum conversion Time = N bits d. Dual Slope iv. Uses a DAC in its feedback patd Codes :
Important Questions from Analog To Digital Converters
Which is known as flash converter?
A 200 mV full-scale dual-slope analog to digital converter (DS-ADC) has a reference voltage of 100 mV. The first integration time is set as 100 ms. The DS-ADC is operated in the continuous conversion mode. The conversion time of the DS-ADC for an input voltage of 123.4 mV (in ms, rounded off to one decimal place) is
- The minimum number of comparators required in a 6-bit flash analog-to-digital converter is ________. (answer in integer)
- The number of comparator required for a 5 bit flash ADC is ____________.
In successive approximation converter input to the comparator is through
